The radio switched to sombre classical music, and theatres, cinemas, and restaurants closed when the news of the fall of Dien Bien Phu reached France on May 7 1954. Teachers with tears in their eyes interrupted their lessons to address half-comprehending pupils on the great tragedy that had overtaken their country. Something terrible, irrevocable and very important had happened in a remote and muddy valley thousands of miles away. The battle which took place there profoundly changed the lives of three nations, and altered the history of many other states.

The Vietnamese communist belief that their final victory was inevitable was greatly reinforced, while France went on to another febrile test of will in Algeria, one which almost precipitated civil war at home. In the United States, those who later made the decision for war saw Dien Bien Phu as an indication of French weakness rather than of Viet Minh strength. It was precisely the wrong conclusion to draw, and one for which the Americans and the peoples of Indochina were all to pay dearly.

Dien Bien Phu broke the already faltering resolve of the French government and people to persevere in Indochina. The French were gone from what became North Vietnam four months later, and out of South Vietnam by the spring of 1956. But Dien Bien Phu, as Martin Windrow's moving and judicious account of the battle and of the two armies that fought it shows, was not at all a straightforward clash between the French and the Vietnamese. Fewer than 20% of the forces engaged on the French side were French, the rest being Vietnamese, North African, West African, and Foreign Legion, mainly German. Nor was the battle fought in a region populated by civilians mainly supportive of the Vietnamese communist soldiers, the famous ocean to their fish.

The people of the high country, in which so many of the decisive engagements of the Vietnamese wars took place, are not Vietnamese but belong to diverse ethnic minorities. In the main they leaned towards the French and many fought with them. Finally, it was not a contest of the kind familiar from the American years, between a western force with greatly superior technical resources and an indigenous army making up for its lack of equipment with courage and willpower, although the People's Army certainly displayed both those qualities. It was much more evenly matched than that.

The French-led forces in Indochina were held together by string and sealing wax. Ill-paid professionals rather than conscripts, they were weak in manpower, madly juggling their best forces round by road and air to stem an advance here, extricate a unit there, or, in yet another location, to seize some opportunity which suddenly presented itself. They were weak in equipment, much of it distinctly secondhand, and where it was new, inexperienced and inadequately trained in its use, particularly the case with aircraft.

Although they got more from the Americans during the fight for Dien Bien Phu, it was never enough, and what they did get they found hard to absorb. They could only dream, for instance, of the kind of precise air support which later became a commonplace in Vietnam. They were hugely vulnerable to attack on the roads, and their only big asset, as Windrow makes clear, was their ability to insert first-class infantry, along with some supporting artillery and some general air support, into places where the People's Army and its commander, Vo Nguyen Giap, did not expect them.

Nor were their weaknesses only material. Windrow's fine portrait of the French forces charts the divisions between those who had been with the Free French from the start, those Vichy units in Africa who rallied after the Allied landings, those which came over even later or not at all, and all these on top of the existing distinctions between France's metropolitan, African, and Far Eastern troops. Some of the French generals were charismatic, most were competent, but also competitive and even quarrelsome. On the other side, the People's Army had the advantage in manpower, the support of the populace in many but not all parts of the Vietnamese lowlands, and much more equipment than is sometimes imagined, including, critically, the medium and heavy artillery, and the anti-aircraft guns, which Giap dug in around Dien Bien Phu. Giap him self, as Windrow says, was a general of limited talents but great determination and ruthlessness.

By 1953, the French knew they had lost the war against the Viet Minh in the north of the country. By the end of the year, their hopes centred on achieving enough in the way of military success to give them some leverage in negotiations. These would concede territory to the communists but allow the French to preserve their position in what was to become South Vietnam, as well as in Laos and Cambodia. When the USSR suggested in late November that representatives of Britain, France, America and the Soviet Union meet in Geneva in April 1954, it was at once clear that this would be the decisive forum for such negotiations.

French paratroops had been dropped into Dien Bien Phu only a week before and were busy establishing an "air ground base". Its purpose was to be an obstacle to any threat to Laos by Giap's forces, and to support the guerrilla forces the French had raised among the high country people. The distant model was of Wingate's Chindit operations in Burma. The more immediate precedent was constituted by a number of succesful French air-ground efforts, notably at Na San, also in the High Region, a year before. Giap's relatively new conventional divisions could be tripped up and deflected by forces operating from such bases, it was believed, and if his troops concentrated to deal with the base, they could be savaged by French guns and aircraft.

Giap might have ignored the provocation or dealt with it in a more limited way. But the prospect of a Geneva meeting transformed the situation. Here was a potential victory, or at least a way to deny victory to the other side, which could tip the balance in negotiations. He began to bring forward his divisions, guns, and supplies, and thus the battle lines were drawn. What followed was an epic of endurance by defenders and attackers alike. Windrow tells it with great lucidity and humanity, with vivid portraits of the leading French officers, and with what seems to be a very fair assessment of the internal dramas of Dien Bien Phu, including the controversial question of whether the senior officer, Christian de Castries, delegated command to energetic deputies, or had it taken from him. He also offers asides on weaponry, battlefield medicine, morale, and other topics which are among the best elements in this good book.

All but the most expert of readers will sometimes lose track of the increasingly fragmented French units and sub-units as they cross and recross their shrinking domain in response to attacks. But all will retain the extraordinary impression of what military work is like in these circumstances, day after day of shelling, attacks and counter-attacks, deaths and injuries briefly recorded, heroism or failure barely noted, before (after repairs to both men and weapons) the whole grim process starts again. The same detail is not given for the communist side, presumably because it is not available, either in translation or in Vietnamese, to the same extent. But Giap once said that every time you leave your house in North Vietnam "you meet a hero", and there were plenty of those, misguided or otherwise, on both sides in the valley of Dien Bien Phu.